Computed tomography as an extension of classical methods in the analysis of soil compaction, exemplified on samples from two tillage treatments and at two moisture tensions
Introduction
Soil physical properties such as dry bulk density, aggregate stability or water retention offer valuable information on pore morphology and can easily be measured using classical laboratory methods. However, explicit information about soil structure changes cannot be obtained with classical methods non-destructively. This requires non-destructive measurements such as X-ray computed tomography (CT), a non-invasive 3D imaging technique. This allows to analyse the morphology of the soil pore system at a given resolution in great detail (Keller et al., 2013; Pöhlitz et al., 2018; Rabot et al., 2018; Schlüter et al., 2018).
A promising application is to investigate structural changes induced by soil compaction in agricultural soils which is associated with different tillage systems and different soil moisture regimes. Soil compaction causes serious environmental and agricultural problems, e.g. soil erosion and flooding due to increased runoff, water quality problems due to the deposition of eroded material in water bodies, and yield reductions due to the loss of fertile topsoil and the restriction of root growth. More directly, compaction has a negative impact on soil aeration and the water balance. The primary cause of soil compaction is the repeated application of heavy loads on the field in the process of agronomic operations such as tillage, seeding, fertilization, crop protection and harvest.
The task of soil tillage is to support crop development by improving gas exchange and the water balance. There is an increasing focus on conservation-oriented soil tillage techniques (e.g. Amin et al., 2014; Dal Ferro et al., 2014) and the risk of damaging soil structure can be minimized (Koch et al., 2008; Rücknagel et al., 2012a). For example, farmers can combine individual working steps to reduce the number of times the field is driven over (Pagliai et al., 1983). At the same time it is necessary to consider that soil compaction of conservation-oriented tillage especially no tillage systems is more crucial to restrict soil functions.
Differences in soil tillage practices are mainly reflected in structural attributes of the topsoil. Physical soil properties such as dry bulk density, aggregate stability or pore size distribution are directly influenced by mechanical disturbances during tillage (FAO, 1993), and by driving over the ground (Carter, 2004). The change in physical properties is associated with changes in the morphology of the pore system which can affect the air, heat and water balance to varying degrees (e.g. Imhoff et al., 2004; Pagliai and Jones, 2002; Saffih-Hdadi et al., 2009).
Mechanical stability of soil is highly sensitive to soil moisture (Rücknagel et al., 2012b). This is well known and has been studied intensely. Soil moisture is not only subject to seasonal fluctuations, but also affected by soil tillage and crop type (Khan et al., 1999; Mackie-Dawson et al., 1989). Thus, even well suited soil tillage systems can massively damage soil structure depending on the actual conditions in terms of soil moisture. Many previous studies have extensively dealt with the effect of different tillage systems on soil compaction (e.g. Dal Ferro et al., 2014; Jarvis et al., 2017), with the interactions between mechanical precompression stress, dry bulk density and water content (e.g. Alexandrou and Earl, 1998), and with models and predictions of mechanical precompression stress for different water content values (e.g. Rücknagel et al., 2012b; Saffih-Hdadi et al., 2009).
In this paper we investigate what can be gained, if computed tomography is used in conjunction with classical methods to describe soil compaction. To do so, the effects of increasing stress on soil samples from two tillage treatments (cultivator and plough) and at two moisture regimes (6 and 1000 kPa) were investigated by looking at classical physical and mechanical soil properties, as well as at the pore structure examined with X-ray CT.
We are well aware that the relationship between compaction, water content and tillage, respectively, has already been extensively researched. Our motivation is to explore the added value of 3D imaging for understanding the pore scale processes during compaction for the different tillage systems and moisture regimes.
Section snippets
Trial site
The site is located near Buttelstedt (Germany, federal state of Thuringia, 11° 20′ O, 51° 4′ N; 200 m above sea level), where a tillage trial was established in 2008. The average annual temperature is 8.4 °C, the average annual precipitation amounts to 541 mm and the average annual evapotranspiration to 641 mm. The soil type is a Chernozem (FAO, 1998), the texture of the topsoil (0–30 cm) is silty clay loam (USDA, 1997) with 293 g kg−1 clay, 662 g kg−1 silt and 45 g kg−1 sand. The total organic
Initial soil physical conditions
As one would expect from the research cited in the introduction, the plough treatment initially had a lower BD (1.19 g cm−3) and a higher saturated conductivity (530.3 cm d−1) compared to the cultivator treatment (1.48 g cm−3, 47.9 cm d−1). The gravimetric water content was significantly higher for ‘plough’ than ‘cultivator’ at −6 kPa. However, at −1000 kPa matric potential the water content did not differ significantly between the samples taken for the two load step application methods (Table 1
Effects of tillage treatment and matric potential on compaction
Our study confirms older work on the influence of tillage and soil moisture on mechanical stability (e.g. Imhoff et al., 2004; Saffih-Hdadi et al., 2009), namely a mechanically more stable soil structure under cultivator compared to plough. There was a disproportionately high increase in mechanical stability in the ploughed soil for a dry soil moisture regime. This increase was found to a much lesser extent in the cultivator treatment.
Aggregates have a much larger recompression range and
Conclusions
With X-ray tomography (CT) it was possible to directly analyse the morphology of soil structure during compaction. In contrast to classical parameters, by using CT, a disturbance of the soil during the measuring process is reduced. The shown CT parameters can be used for a standardized characterization of the soil. Hence, CT provides a more complete picture of the compaction effects.
The combination of CT analysis with classical soil mechanical measurement provided a consistent interpretation
Acknowledgments
Many thanks to K. Marschall from the “Thüringer Landesanstalt für Landwirtschaft” and S. Reimann from the “Thüringer Lehr-, Prüf- und Versuchsgut GmbH” for the permission to take soil samples at their experimental site in Buttelstedt. Also, the expert technical assistance and guidance of J. M. Köhne when it came to CT image acquisition is gratefully acknowledged.
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